try

{

可能會(huì)出現(xiàn)異常的代碼段；

}

catch(異常類型名 處理該異常對(duì)象)

{

異常處理代碼段；

}

 1 import java.io.*; 2  3 public class TryCatchTest { 4  5     public static void main(String[] args) { 6         File file = new File("abc.txt"); 7         int a[] = {1, 2}; 8          9         try10         {11             System.out.PRintln(3/0);12         }13         catch(ArithmeticException e1)14         {15             System.out.println("3/0: ");16             System.out.println("This is ArithmeticException");17         }18         19         try20         {21             System.out.println(a[2]);22         }23         catch(ArrayIndexOutOfBoundsException e2)24         {25             System.out.println("a[2] is out of Array: ");26             System.out.println("This is ArrayIndexOutOfBoundsException");27         }28         29         try30         {31             BufferedReader input = new BufferedReader(new FileReader(file));32         }33         catch (FileNotFoundException e3)34         {35             System.out.println("abc.txt is not found: ");36             System.out.println("This is FileNotFoundException");37         }38         catch(IOException e)39         {40             System.out.println("This is IOException");41         }42 43     }44 45 }

3/0: This is ArithmeticExceptiona[2] is out of Array: This is ArrayIndexOutOfBoundsExceptionabc.txt is not found: This is FileNotFoundException

編寫代碼過(guò)程中，如果不想在這段代碼中捕捉和處理一個(gè)可能出現(xiàn)的異常，那么就需要將這個(gè)異常傳遞出去，傳遞給調(diào)用它的方法去處理該異常。這個(gè)時(shí)候就需要使用throw 和throws

• throws語(yǔ)句在方法聲明中使用，拋出異常
• throw語(yǔ)句在方法體內(nèi)部使用，拋出異常

注意： 方法體中若使用了throw語(yǔ)句拋出異常，則必須在該方法聲明中，采用throws語(yǔ)句來(lái)聲明該方法體中拋出的異常，同時(shí)，throws語(yǔ)句聲明拋出的異常，必須是方法體中throw語(yǔ)句拋出的異常或該異常的父類。

 1 import java.io.*; 2  3 public class ThrowTest { 4      5     public void throwTest1() throws ArithmeticException 6     { 7         System.out.println(3/0); 8     } 9     10     public void throwTest2() throws ArrayIndexOutOfBoundsException11     {12         int a[] ={1,2};13         System.out.println(a[2]);14     }15     16     public void throwTest3() throws FileNotFoundException17     {18         File file=new File("abc.txt");19         new BufferedReader(new FileReader(file));20     }21     22     public void throwTest4() throws FileNotFoundException23     {24         throw new FileNotFoundException("abc.txt");25     }26 27     public static void main(String[] args) {28         ThrowTest throwTest=new ThrowTest();29         30         try31         {32             throwTest.throwTest1();33         }34         catch (ArithmeticException e1)35         {36             System.out.println("3/0: ");37             System.out.println("This is ArithmeticException");38         }39         40         try 41         {42             throwTest.throwTest2();43         }44         catch(ArrayIndexOutOfBoundsException e2)45         {46             System.out.println("a[2] is out of Array: ");47             System.out.println("This is ArrayIndexOutOfBoundsException");48         }49         50         try51         {52             throwTest.throwTest3();53         }54         catch (FileNotFoundException e3)55         {56             System.out.println("abc.txt is not found: ");57             System.out.println("This is FileNotFoundException");58         }59         60         try61         {62             throwTest.throwTest4();63         }64         catch (FileNotFoundException e3)65         {66             System.out.println("abc.txt is not found: ");67             System.out.println("This is FileNotFoundException");68         }69 70     }71 72 }

3/0: This is ArithmeticExceptiona[2] is out of Array: This is ArrayIndexOutOfBoundsExceptionabc.txt is not found: This is FileNotFoundExceptionabc.txt is not found: This is FileNotFoundException

建立自己的異常類，要做的只是根據(jù)需要，從Exception類或者從Exception類的子類中繼承出需要的類。習(xí)慣上，會(huì)經(jīng)常為每一個(gè)異常類，提供一個(gè)默認(rèn)的和一個(gè)包含詳細(xì)信息的構(gòu)造器。需要注意的是，自定義異常類，必須由程序員使用throw語(yǔ)句拋出。

 1 public class MyException { 2  3     public static void main(String[] args) { 4         String str="2abcde"; 5          6         try 7         { 8             char c=str.charAt(0); 9             if(c<'a'||c>'z'||c<'A'||c>'Z')10                 throw new FirstLetterException();11         }12         catch (FirstLetterException e)13         {14             System.out.println("This is FirstLetterException");15         }16 17     }18 19 }20 21 class FirstLetterException extends Exception{22     public FirstLetterException()23     {24         super("The first char is not a letter");25     }26     27     public FirstLetterException(String str)28     {29         super(str);30     }31 }

This is FirstLetterException

 1 public class MyException { 2  3     public static void main(String[] args) throws FirstLetterException{ 4         throw new FirstLetterException(); 5     } 6 } 7  8 class FirstLetterException extends Exception{ 9     public FirstLetterException()10     {11         super("The first char is not a letter");12     }13     14     public FirstLetterException(String str)15     {16         super(str);17     }18 }

Exception in thread "main" FirstLetterException: The first char is not a letterat MyException.main(MyException.java:5)

在使用try...catch語(yǔ)句是，若try語(yǔ)句中的某一句出現(xiàn)異常情況，那么這部分try語(yǔ)句段中，從出現(xiàn)異常的語(yǔ)句開始，之后的所有語(yǔ)句都不會(huì)被執(zhí)行，直到這部分try語(yǔ)句段結(jié)束。

但是在很多情況下，希望無(wú)論是否出現(xiàn)異常，某些語(yǔ)句都需要被執(zhí)行。那么就可以把這部分代碼放在finally語(yǔ)句段中，即使try或catch語(yǔ)句段中含有return語(yǔ)句，程序都會(huì)在異常拋出后先執(zhí)行finally語(yǔ)句段，除非try或catch語(yǔ)句段中執(zhí)行System.exit()方法，或者是出現(xiàn)Error錯(cuò)誤，finally語(yǔ)句才不會(huì)被執(zhí)行而退出程序。

 1 import java.io.*; 2  3 public class FinallyTest { 4  5     public static void main(String[] args) { 6         File file=null; 7         BufferedReader input=null; 8         file=new File("abc.txt"); 9         10         try11         {12             input=new BufferedReader(new FileReader(file));13         }14         catch(FileNotFoundException e)15         {16             System.out.print("abc.txt is not found: ");17             System.out.println("This is FileNotFoundException");18         }19         finally20         {21             System.out.println("This is finally code part.");22         }23 24     }25 26 }

abc.txt is not found: This is FileNotFoundExceptionThis is finally code part.